Since shortly after the discovery of X-rays, an X-ray conversion screen, also called an X-ray intensifying screen, has been used to convert X-ray images to visible or near-visible images. The key constitutent of an X-ray conversion screen is a phosphor material which absorbs incident X-ray photons and produces in their stead photons of visible or near-visible energy. Such screens are now used widely in industry and medicine. In use, the screen, mounted in a cassette, is placed directly in the X-ray beam and comes into immediate contact with a sheet of photosensitive film which is more sensitive to the light emitted by the phosphor screen than to the X-rays. Thus, an "intensified" image is produced on the film.
X-ray quality is commonly measured in terms of the voltage applied to the X-ray tube which generates the X-rays. For medical use, X-rays are produced at tube potentials of about 30-140 killivolts (kVp). When the phosphor contained in the X-ray screen is struck by X-rays, a certain amount of the energy of the X-radiation is absorbed by the phosphor. The amount of energy absorbed--i.e., the efficiency of the phosphor--will vary from phosphor to phosphor. Since the phosphor emits visible light (fluoresces) in proportion to the amount of X-ray energy absorbed, and since it is this visible light which exposes the photosensitive film, a highly efficient phosphor system is desirable. Also, X-rays are applied from the X-ray source at varying kilovoltage of output--known as the kVp spectrum. To be highly useful, an X-ray screen must respond adequately at the high and low ends of the kVp spectrum.
Thus, to be useful, the X-ray phosphor material must stop X-rays effectively and must convert efficiently the absorbed X-ray energy into light photons. Thus, the ideal phosphor material should have both a high X-ray absorption coefficient and a high X-ray conversion efficiency.
Using green-sensitive X-ray film, patient exposure can be reduced if all components of a phosphor mixture emit in the green (to which the film is most sensitive) as compared to a mixture in which one component emits blue (to which the film is not sensitive).
Three phosphors generally utilized in X-ray conversion screens are: calcium tungstate (CaWO.sub.4), and cesium iodide (CsI), cadmium-zinc sulfide powders, typically (Cd.sub.0.5 Zn.sub.0.5)S:Ag. The first two phosphors, however, have the disadvantage that they emit light principally in the blue region of the spectrum, and the third phosphor emits in the yellow-green.
U.S. Pat. No. 3,725,704 describes an X-ray conversion screen which employs a phosphor consisting essentially of at least one oxysulfide selected from the group consisting of lanthanum oxysulfide, gadolinium oxysulfide and lutetium oxysulfide, in which from about 0.005 percent to 8 percent of the host metal ions have been replaced by trivalent terbium ions. Conversion screens utilizing one of the phosphors of that invention, when placed in an X-ray beam, convert X-ray photons to radiation in the blue and green portion of the visible spectrum, principally in the green portion, between about 500 and 600 nm.
U.S. Pat. No. 3,738,856 is directed to an X-ray conversion screen which employs a phosphor consisting of, in one embodiment, a 95/5 mixture of gadolinium and yttrium oxysulfide, activated with terbium. An analogous phosphor, gadolinium tantalate activated with terbium, is disclosed in U.S. Pat. No. 4,225,653.
U.S. Pat. No. 4,259,588 teaches the use of an absorber, i.e., a yellow dye, mixed in with a green-emitting X-ray phosphor, e.g., terbium-activated gadolinium oxysulfide, in an X-ray intensifying screen. It is claimed that the yellow dye absorbs trace amounts of blue emission given off naturally by the phosphor, thus improving certain characteristics.
None of the above phosphor systems exhibit improved absorption over a broad range of the kVp system.